1. Field of the Invention
The invention relates to an air turning bar for a take-off device for plastic films, in which air outlet openings are provided in that region of its circumferential surface around which the plastic film is wrapped during its transportation.
2. Description of the Prior Art
In the case of an air turning bar which is disclosed in EP 0 992 334 A2, in order to produce an air cushion in the wrapping region of the plastic film, the said air turning bar is bent over its entire length and is set in such a manner that the concave side is situated essentially at the vertex of the deflection. This produces a relatively large cavity between the film and the air turning bar, a lot of compressed air being required in order to fill the said cavity. Furthermore, the bending of the air turning bar means that the side borders of the film have only a relatively small border region which is suitable for sealing the air cushion. Furthermore, it is difficult to produce uniform bending of the air turning bar. Finally, the bending of the air turning bar means that, during an adjustment in the circumferential direction, problems may occur in an undesirable manner because of contact between the film and the air turning bar.
The object of the invention is to provide an air turning bar, in which an air cushion region which can be delimited precisely can be provided without the disadvantages of a bent air turning bar in terms of its production and its limitations having to be accepted during an adjustment in the circumferential direction.
This object is achieved according to the invention in the case of an air turning bar of the type mentioned at the beginning in that the air turning bar has, at least in one part of the wrapping region, a depression with respect to the remaining circular cylindrical circumference.
A depression in the air turning bar can be adapted to the particular requirements without a deviation from the circular cylindrical shape of the air turning bar being present in the remaining region, which deviation could initiate possible disturbances with regard to the surrounding plastic film. Furthermore, the insertion of a depression can be brought about more simply than the bending of an entire bar, and, specifically, in a predetermined, exact shape. The depression enables the air cushion space to be designed in accordance with the requirements, which is not possible to this extent with bending of the air turning bar, because the air cushion space has inevitable, different dimensions in thickness along the air turning bar due to the bending. The depression can be adapted in its shape and depth to the particular conditions without the remaining circumference of the air turning bar being influenced.
In a development of the invention, the depression can be produced by pressing in a hollow air turning bar, or there is also the possibility of producing the depression by a recess.
A further, advantageous refinement of the invention consists in the air turning bar having, at the ends of the depression, as seen in the longitudinal direction of the air turning bar, a respective strip having a circular cylindrical circumference. In contrast to a bent air turning bar, this enables a relatively great resistance to be set in the border region against air flowing off in the longitudinal direction of the air turning bar and therefore enables leakage losses to be reduced. Depending on the requirements present in each case, the depression may have, at its border, a rounded transitional region to the circular cylindrical region or the depression may be bounded at its border by a sharp edge. These design possibilities are not provided in the case of an air turning bar bent in its entirety.
The depression may have at least one row of air outlet openings orientated in the longitudinal direction of the depression or a plurality of air outlet openings may be distributed over the surface of the depression, which can depend on the shape of the depression, the size of the air outlet openings, the width of the plastic film to be taken off, etc.
If, in a development of the invention, the air turning bar can be rotated and secured in the circumferential direction, then the position of the depression with respect to the plastic film, and therefore the influence of the air cushion on the plastic film, can be changed.